Viscosity control of formulations relating to various personal care, oral care, household and institutional applications in an important attribute for consumer use of such products.
Several approaches to control viscosity of such formulations are known to the art. Typically, a certain amount of a high molecular weight synthetic or natural polymers, such as, for example, linear or cross-linked acrylic acid based polymers, xanthan gum, various cellulose derivatives or other polysaccharide derivatives is incorporated into the formulation to impart a desired rheology.
Rheology delivered by these high molecular weight synthetic or natural polymers is usually strongly shear-thinning exhibiting high viscosity at low sheer rates, but relatively low viscosity at high shear. Such formulations usually do not exhibit a Newtonian or shear independent viscosity plateau, or if these formulation do exhibit Newtonian plateau, it is at shear rates below 1 s−1.
A common and inexpensive method of delivering viscosity to formulations is through the addition of salts such as, for example, sodium chloride, sodium sulfate or ammonium chloride to the formulations. Addition of such salts in amounts ranging from between 0.1 to 5 wt % in cleansing formulations containing surfactants such as for example, sodium lauryl or ammonium lauryl sulfate, result in cleansing formulations with increased viscosity. One advantage of the use of salt to thicken formulations is that the resultant thickened cleansing formulation may be relatively clear.
Salt thickened formulations are commonly used and exhibit characteristic rheological properties. The characteristic rheological properties of these salt thickened formulations can be described as exhibiting shear independent (or Newtonian) viscosities up to a shear rate of the order of about 10 to 100 s−1 followed by a decrease in its viscosity as the shear rate is increased above 100 s−1. This phenomenon is known as “sheer-thinning”. The salt thickening of formulations, however, has an important drawback in that the efficiency of salt to thicken a formulation decreases rapidly as the amount of surfactant contained in the formulation decreases.
A need exists for surfactant-based aqueous formulations exhibiting Newtonian viscosity at lower sheer rates and sheer thinning at higher sheer rates while permitting the use of various amounts of surfactants, including lower surfactant amounts.